How to scale vertices in only top view but not impacting how they look in front view?
select the vertices, edges or faces you want to scale
use axle restrictions.

press to individually restrict:


x
Y
z


If you wish to restrict 2 axes, press:


SHIFT + Z to scale in X, Y, and restrict Z
SHIFT + X to scale in Y, Z, and and restrict X
SHIFT + Y to scale in X, Z, and restrict Y